1. Field of the Invention
The present invention relates to a sensor apparatus having a detecting unit such as a force sensor and a torque sensor and a robot apparatus having the sensor apparatus, and more particularly to a robot apparatus for positioning in a stable manner and at a high speed.
2. Description of the Related Art
In general, the assembling robot apparatus includes a robot arm body having a plurality of links and a hand, in which the links are swingably coupled to each other through a joint portion. The joint portion includes a servomotor and positioning control is performed by driving the servomotor.
This type of robot apparatus has a torque sensor detecting a joint torque acting on a link. Based on the detected torque value, the robot apparatus generates a torque command value to control the servomotor (see Japanese Patent Application Laid-Open No. H08-118275). The torque sensor is configured to include a flexible member having a plurality of beams radially extending from a center portion and a ring-shaped frame connecting the beams; and a sensor detecting a deformation amount (deflection amount) of the flexible member. The torque sensor outputs a detection signal indicating a torque corresponding to the deformation amount of the flexible member.
Further, the robot apparatus includes a force sensor detecting a force applied to a hand when the hand grips an object to be gripped. The force sensor includes a flexible member which deforms according to the force applied to the hand by the object gripped by the hand; and a sensor detecting the deformation amount of the flexible member. The force sensor is configured to output a detection signal indicating a result of detection by the sensor.
Unfortunately, in the robot apparatus having the detecting unit such as the force sensor and the torque sensor, the robot arm body vibrates at a natural vibration frequency and thus the flexible member of the detecting unit also vibrates at the natural vibration frequency. Thus, the vibration of the flexible member causes the signal component of an output original detection signal to include a noise component vibrating at the natural vibration frequency. A control of each servomotor using the original detection signal including the noise component reduces the accuracy of positioning control on the hand and each link.
FIGS. 14A to 14D are graphs for describing the original detection signal from the detecting unit. As illustrated in FIG. 14A, it is found that a noise component N due to vibration of the robot arm body occurs in a frequency band higher than the frequency of a signal component S. In light of this, it can be considered to use a filter which removes a specific frequency component f1 from the original detection signal of the detecting unit to output the detection signal. If the frequency f1 of the noise component N vibrating at the natural vibration frequency does not change, the use of this filter can remove the noise component N from the original detection signal.
Unfortunately, the frequency of the noise component N due to the natural vibration frequency changes according to the weight of the object gripped by the hand. For example, as illustrated in FIG. 14B, the frequency may change to a frequency f2 different from the frequency f1. Consequently, if the frequency changes, this filter cannot remove the noise component N.
In light of this, it can be considered to apply a low pass filter having a frequency characteristic as illustrated in FIG. 14C of attenuating a frequency band W in a possible range of the noise component N in order to remove the noise component in the natural vibration frequency with changeable frequency from the original detection signal output from the detecting unit. Since the frequency component with a cutoff frequency fc or less has a low attenuation, the use of this filter allows the signal component S of the detection signal to pass and the noise component N to be cut off even if the frequency of the noise component N changes in the frequency band W higher than that of the signal component S.
In order to use this filter, the cutoff frequency fc needs to be set as low as possible so as to correspond to frequencies f1 and f2 in which the noise component N changes. Consequently, the signal component S of the detection signal exists in the vicinity of the cutoff frequency fc.
Unfortunately, as illustrated in FIG. 14D, the filter has a phase-frequency characteristic. Consequently, a phase shift occurs in a frequency band in the vicinity of the cutoff frequency fc of the filter. Thus, a phase delay always occurs in the signal component S of the detection signal subjected to filtering. As a result, a phase-delayed detection signal S is used to control the servomotor, leading to a delayed response to the target.
Thus, it is an object of the present invention to well remove a noise component due to vibration of a flexible member from an original detection signal output from a detecting unit and suppress a phase delay of a detection signal obtained by filtering.